1. Field
This patent application relates generally to control and containment of gases and more specifically to preventing back-flash in a scrubbing apparatus.
2. Description of Related Art
A variety of industrial processes create gas streams that must be scrubbed of contaminants before being released to the outside world. The manufacture of electronics, solar cells, display devices, communications devices, metals, ceramics, and polymers, as well as the processing of chemicals, drugs, and other materials, often requires the use of exhaust gas scrubbers. Scrubbers typically receive a substantially gaseous exhaust stream (sometimes containing fine particles) and remove contaminants from the gas stream before the stream is released to the environment.
Exhaust streams from electronic fabrication processes may include a variety of contaminants, including but not limited to perfluorocarbon (PFC) etch gases such as SF6, NF3, CF4, C2F6, C4F8, COF2, and C4F6. Exhaust streams may include toxic hydrides such as AsH3, PH3, P2H4, or B2H6. Exhaust streams may also contain pyrophoric or flammable gases such as SiH4, H2, Si2H6, GeH4, and gases such as WF6, SiF4, HCl, BCl3, Cl2, TiCl4, F2, HF, and various chlorosilanes.
Other industrial processes may also create toxic or polluting exhaust streams particular to a material or manufacturing process. Volatile organic compounds (VOCs) may be present in various petroleum refining processes, chemical reaction processes, or other organic synthesis reactors. Room or chamber ventilation (e.g., of a spray painting facility or an environment containing microbes or viruses) may also require exhaust gas scrubbing or the use of other abatement systems.
Many contaminants require specific scrubbing procedures. Contaminants such as HCl, Cl2, and BCl3 are often soluble in water, and may often be removed using so-called wet scrubbers. Contaminants such as SiCl4, SiH2Cl2, NH4F, WF6, WCl4, and TiCl4 (herein “water-reactive” contaminants) may or may not dissolve in water, depending upon various conditions. These contaminants may also react with water to form solid reaction products, which may clog various flow paths.
Another category of contaminants includes “water-insoluble” contaminants such as SiH4, PFCs such as CF4 and C2F6, SF6, and NF3. Among other deleterious characteristics, many of these contaminants are characterized by a “global warming potential,” which may be hundreds or thousands of times stronger than that of CO2 and reflecting a much stronger behavior as a greenhouse gas in the Earth's atmosphere.
Some contaminants are often abated by combusting the contaminant to form water-soluble reaction products that are then removed by wet scrubbing. Sometimes, such combustion requires high temperatures. For example, NF3 may be combusted at temperatures above 900 degrees Celsius; CF4 may be combusted at temperatures over 1200 degrees Celsius. Other contaminants such as SiH4 may sometimes be reacted simply by exposing the contaminant to an oxygen source.
Water-insoluble, thermally decomposed contaminants may form reaction products (e.g., HF) that may be removed by wet scrubbing the reacted gas stream. Other water-insoluble contaminants (e.g., SiH4) may form reaction products that include solid species (e.g., SiO2), when thermally reacted.
Generally, solid species in a waste stream may be present as fine particles liquid phase (e.g., water associated with a scrubber), in the phase, deposited on a solid surface, or in other ways. These solid species may also nucleate directly on various surfaces. While the formation of solid reaction products may enable certain removal methods (e.g., filtration), these species may also deposit on and clog various lines, inlets, passages, surfaces, and other aspects of the system, reducing the system's efficiency or stopping its operation.
Previous designs used separate process chamber inlets organized such that the mixing of reactive gases occurs at or very near the thermal abatement zone. This prevents the reactive gases from combining and reacting outside of the thermal abatement zone in a reaction chamber.
Thermally decomposing noxious or flammable gases can pose flammability or explosion hazards. The flame front within a reaction chamber can propagate upstream into chambers that are not built to contain the pressures of an explosion or into materials that are not suitable for contact with high temperatures or flames. If the velocity of the flame front is greater than the velocity of the gas entering the apparatus, the flame migrates upstream. This can burn interior surfaces of plumbing or devices or cause an explosion. To combat this phenomenon, prior designs have used constricted inlets. By inserting one or multiple constrictions in the incoming flow stream, the velocity of the gas is increased through the constriction to the point where the flame front cannot migrate upstream. If the constriction is of fixed impedance and the flow increases above the calculated range of operation, higher pressure results upstream of the inlet. If the flow decreases to where the flame front velocity is greater than the flow velocity, the flame will migrate upstream through the constriction and thereby causing a hazard. With a variable constriction, these issues can be overcome; however, control and monitoring systems are complicated and subject to failure and thus are not safe. Another prior device is a backflash arrestor. This device may consist of a metal mesh or baffle installed in the inlet pipe. As the flame propagates upstream it encounters the mesh or baffle whereby it is cooled to below the flammable temperature limit and extinguished. However, such devices are prone to clogging and corroding requiring excessive maintenance.